This invention relates to combination water and space heating systems, and in particular, to domestic water heaters used with circulating boilers.
Combination space heating and domestic hot water systems fall into two broad categories: open loop systems and closed loop systems. In an open loop system, a boiler or hot water heater produces potable hot water that is also used for heating air to satisfy the space heating requirements. It is characteristic of such systems that the boiler circulates a constantly changing supply of water as potable hot water draws are made from the system and cold supply water replaces it. It is also characteristic of such systems that the water circulated for the purpose of space heating must have the same temperature as that of the potable hot water. A difficulty with such systems is that large reservoirs or tanks of hot water are required to satisfy the demand for short term energy requirements, and such large reservoirs have high inherent energy losses.
In the closed loop system, the boiler loop is separated from the domestic hot water system, and an unchanging supply of fluid is circulated in the boiler loop. In a closed loop combination system, domestic hot water is generated by a heat exchanger whose function it is to maintain physical separation between the circulating boiler fluid and the domestic water supply. A closed loop system is somewhat more complex than an open loop system, but it offers the advantage that the boiler loop can operate at a higher temperature than the domestic hot water system.
The advantage of operating the boiler loop at a higher temperature (say 190xc2x0 F.) is that if radiators or convectors are used for space heating, less heat transfer area is required to move a given amount of heat energy than if the boiler loop is limited to normal domestic hot water temperature about 140xc2x0 F. (60xc2x0 C.).
There have been several approaches to heat exchanger design for generating domestic hot water in closed loop combination systems. These approaches can be broadly categorized as follows:
1. Storage tank water heaters
2. Instantaneous water heaters
3. Semi-instantaneous water heaters.
In the first approach, a heat exchanger is immersed in a relatively large tank that supplies the domestic hot water. Hot water demand is met largely by stored capacitance. One advantage of the storage tank water heater is inherent temperature stability in the hot water supply due to the large thermal capacitance of the stored hot water. Another advantage is that a large flowrate may be tapped, at least until the tank is drained of hot water and the boiler cannot keep up with the demand. The disadvantage of this approach is similar to the open loop system described above, in that a large tank must be used, with the associated cost, bulk and thermal losses.
In the instantaneous water heater system, a heat exchanger without any appreciable volume is used. Heat is transferred from the boiler fluid flowing through one side of the heat exchanger to the domestic water flowing through the other side of the heat exchanger. Typically, high fluid velocity is maintained on both sides of the heat exchanger, augmenting the heat transfer coefficient and making possible a compact design relative to the heat transfer rate capacity of the unit. Operationally, the system must have a means to sense hot water draw (a flow switch). A boiler circulation pump and ignition system are energized when water flow is sensed. The advantage of the instantaneous water heater is that no hot water is stored, so that there is no corresponding thermal loss.
A difficulty with the prior art instantaneous water heater systems is that a complicated automatically modulating boiler is mandatory, since there is little thermal capacitance to absorb the energy output of the boiler energy source. The heat energy input to the boiler must closely follow the heat energy output required for the domestic hot water draw. Temperature instability due to rapid changes in the hot water flow rate is inevitable in these systems, and a complex and difficult control system must be provided to try to keep such instability to a reasonable level. Another disadvantage is that the boiler is ill-suited to respond to demand spikes, such as where a hot water tap is opened for a short period and then closed. With the instantaneous water heater, a series of demand spikes causes excessive boiler on/off cycling which is an undesirable operating mode.
In the semi-instantaneous water heater system, a compact forced convection heat exchanger is usually used inside a small storage tank of hot water which provides some thermal capacitance. An example of such a system is shown in U.S. Pat. No. 5,233,970 issued to James A. Harris. This type of system is an improvement over the instantaneous water heater, because the thermal capacitance of the water tank dampens out some of the temperature instabilities associated with the instantaneous water heaters. The thermal capacitance also eases considerably the boiler cycling problem that can arise from demand spikes. A difficulty with the semi-instantaneous system, however, is that a complex and expensive modulating boiler still must be used, again because the small storage tank cannot handle the energy output from a constant output boiler. Also, the system cannot handle short term, high demand loads. There is also undesirable lag time under high demand conditions while the boiler is ramping up its energy output.
In the present invention, an instantaneous heat exchanger is used, but with a boiler with a built in reservoir with sufficient capacity to supply normal short term demand, yet small enough to avoid excessive downtime thermal losses.
According to the invention, there is provided a hot water heater comprising an instantaneous domestic water heat exchanger having a plurality of inner flow passages for domestic water flow therethrough and means defining a domestic hot water inlet and outlet communicating therewith. The heat exchanger also has at least one boiler water passage located adjacent to the inner domestic water flow passages and means defining a boiler water inlet and outlet communicating therewith. A temperature sensor is attached to the heat exchanger to sense the temperature of the domestic hot water therein. A low mass boiler-type heat generator has an internal reservoir defining a boiler water inlet and outlet, a source of heat energy for heating water in the reservoir, and thermostatic means operably connected to the source of heat energy for maintaining the boiler water inside the reservoir between predetermined minimum and maximum temperatures. A boiler water flow circuit connects the heat exchanger and the heat generator reservoir in series. This flow circuit has a supply line for the flow of boiler water from the heat generator reservoir outlet to the heat exchanger boiler water inlet, and a return line for the flow of boiler water from the heat exchanger boiler water outlet back to the heat generator reservoir inlet. Pump means is located in the flow circuit and coupled to the domestic water temperature sensor for causing boiler water flow through the heat exchanger upon the domestic water temperature therein dropping below a predetermined temperature. Also, the heat generator reservoir is of a size large enough to enable the heat exchanger to supply short term domestic water demand and small enough-to avoid excessive downtime energy losses.